Uranium-modified zinc oxide voltage variable resistor

ABSTRACT

A VOLTAGE VARIABLE RESISTOR CERAMIC COMPOSITION CONSISTING ESSENTIALLY OF ZINC OXIDE AND, AS AN ADDITIVE, URANIUM OXIDE. THE URANIUM-MODIFIED ZINC OXIDE VOLTAGE VARIABLE RESISTOR HAS IMPROVED VOLTAGE NONLINEAR PROPERTIES DUE TO THE FURTHER ADDITION OF BISMUTH OXIDE, CALCIUM OXIDE AND COBALT OXIDE.

Patented Oct 17, 1972 3,699,058 URANIUM-MODIFIED Z INC OXIDE VOLTAGE VARIABLE RESISTOR Michio Matsuoka, Takeshi Masuyama, and Yoshio Iida,

Osaka-fu, Japan, assignors to Matsushita Electric Industrial Co., Ltd., Osaka, Japan Filed Oct. 16, 1969, Ser. No. 866,820 Claims priority, application Japan, Oct. 22, 1968, 43/ 77,733 Int. Cl. H01b 1 06 U.S. Cl. 252-518 6 Claims ABSTRACT OF THE DISCLOSURE A voltage variable resistor ceramic composition consisting essentially of zinc oxide and, as an additive, uranium oxide. The uranium-modified zinc oxide voltage variable resistor has improved voltage nonlinear properties due to the further addition of bismuth oxide, calcum oxide and cobalt oxide.

This invention relates to compositions of voltage variable resistor ceramics having non-ohmic resistance and more particularly to compositions of varistors comprising zinc oxide having non-ohmic resistance due to the bulk thereof.

Various voltage variable resstors such as silicon carbide varisitors, selenium rectifiers and germanium or silicon p-n junction diodes have been widely used for stabilization of voltage or current of electrical circuits. The electrical characteristics of such a voltage 'variable resistor are expressed by the relation:

where V is the voltage across the resistor, I is the current flowing through the resistor, C is a constant corresponding to the voltage at a given current and exponent n is a numerical value greater than 1. The value of n is calculated by the following equation:

where V and V2 are the voltages at given currents I and 1 respectively. The desired value of C depends upon the kind of application to which the resistor is to be put. It is ordinarily desirable that the value of n be as large as possible since this exponent determines the extent to which the resstors depart from ohmic characteristics.

In conventional varistors comprising germanium or silicon p-n junction diodes, it is diicult to control the C-value over a wide range because the voltage variable property of these varistors is not attributed to the bulk but to the p-n junction. On the other hand, silicon carbide varistors have voltage variable properties due to the contacts among the individual grains of silicon carbide bonded together by a ceramic binding material, and the C-value is controlled by changing a dimension in a direction in which the current flows through the varistors. Silicon carbide varistors, however, have a relatively low nvalue and are prepared by firing in non-oxidizing atmosphere, especially for the purpose of obtaining a lower C-value.

An object of the present invention is to provide a composition of a voltage variable resistor having non-ohmic properties due to the bulk thereof and having a controllable C-value.

Another object of the present invention is to provide a composition of a voltage variable resistor characterized by a high n-value.

These and other objects of the invention will become apparent upon consideration of the following description taken together with the accompanying drawing in which the single figure is a partly cross-sectional view of a voltage variable resistor according to the invention.

Before proceeding with a detailed description of the voltage variable resstors contemplated by the invention, their Construction will be described with reference to the aforesaid figure of drawing Wherein reference character 10 designates, as a whole, a voltage variable resistor comprising, as its active element, a sintered body having a pair of electrodes 2 and 3 applied to opposite surfaces thereof. Said sintered body 1 is prepared in a manner hereinafter set forth and is in any form such as circular, square or rectangular plate form. Wire leads 5 and 6 are attached conductively to the electrodes 2 and 3, respectively, by a connection means 4 such as solder or the like.

A voltage variable resistor according to the invention comprises a sintered body of a composition consisting essentially of 90.0 to 99.95 mole percent of zinc oxide and 0.05 to 10.0 mole percent of uranium oxide. Such a voltage variable resistor has non-ohmic resistance due to the bulk itself. Therefore, its C-value can be changed without impairing the n-value by changing the distance between said opposite surfaces. The shorter distance results in the lower C-value.

The higher n-value can be obtained when said sintered body consisting essentially of 97.0 to 99.9 mole percent of zinc oxide and '0.1 to 3:0 mole percent of uranium oxide in accordance with the invention.

According to the present invention, the C-value can be lowered without changing the dimension or lowering n-value when said sintered body is of a composition consisting essentially of 82.0 to 99.9 mole percent of zinc oxide, 0.05 to 10.0 mole percent of uranium oxide and 0.05 to 8.0 mole percent of bismuth oxide.

A combination of a low C-value and a high n-value can be obtained when said sintered body consists essentially of 94.0 to 99.8 mole percent of zinc oxide, 0.1 to 3.0 mole percent of uranium oxide and 0.1 to 3.0 mole percent of bismuth oxide.

According to the present invention, the stability for ambient temperature and the electric load life test can be improved when said sintered body consists essentially of 82.0 to 99.9- mole percent of zinc oxide, 0.05 to 10.0 mole percent of uranium oxide and 0.05 to 8.0 mole percent of calcum oxide.

Further, the stability for ambient temperature and the electric load life test is extremely improved when said sintered body consists essentially of 94.0 to 99.8 mole percent of zinc oxide, 0.1 to 3.0*mo1e percent of uranium oxide and 0.1 to 3.0 mole percent of calcum oxide.

According to the present invention, the n-value is elevated when said sintered body consists essentially of 82.0 to 99.9 mole percent of zinc oxide, 0.05 to 10.0 mole percent of uranium oxide and 0.05 to 8.0 mole percent of cobalt oxide.

The n-value is further elevated when said sintered body is of a composition consisting essentially of 94.0 to 99.8 mole percent of zinc oxide, 0.1 to 3.0 mole percent of uranium oxide and 0.1 to 3.0 mole percent of cobalt oxide.

According to the present invention, a combination of a high n-value and a low C-value can be obtained when said sintered body is of a composition consisting essentially of 74.0 to 99.85 mole percent of zinc oxide, 0.05 to 10.0 mole percent of uranium oxide, 0.05 to 8.0 mole percent of bismuth oxide and 0.05 to 8.0 mole percent of cobalt oxide.

Further, the C-'value is lowered and the n-value is extremely elevated when said sintered body is of a composition consisting essentially of 91.0 to 99.7 mole percent of zinc oxide, 0.1 to 3.0 mole percent of uranium oxide, 0.1 to 3.0 mole percent of bismuth oxide and 0.1 to 3.0 mole percent of cobalt oxide.

According to the present invention, a combination of a high nvalue, a low C-value and a high stability can be obtained when said sintered body is of a composition consisting essentially of 74.0 to 99.85 mole percent of zinc oxide, 0.05 to 10.0 mole percent of uranium oxide, 0.05 to 8.0 mole percent of bismuth oxide and 0.05 to 8.0 mole percent of calcium oxide.

Further, a combination of an extremely high n-value at a low C-value and a high stability can be obtained when said sintered body is of a composition consisting essentially of 91.0 to 99.7 mole percent zinc oxide, 0.1 to 3.0 mole percent of uranium oxide, 0.1 to 3.0 mole percent of bismuth oxide and 0.1 to 3.0 mole percent of calcium oxide.

The sintered body 1 can be prepared by a per se well known ceramic technique. The starting materials of the compositions described in the foregoing description are mixed in a wet mill so as to produce homogeneous mixtures. The mixtures are dried and pressed in a moled into desired shapes at a pressure from 100 kg./cm. to 1000 kg./cm. The pressed bodies are sintered in air at a given temperature for 1 to 3 hours, and then furnace-cooled to room temperature (about to about 30 C.).

The available sintering temperature is determined in view of electrical resistivity, non-linearity and stability and ranges from 1000 to 1450 C.

The pressed bodies are preferably sintered in nonoxidizing atmosphere such as nitrogen and argon when it is desired to reduce the electrical resistivity.

The mxtures can be prelirninarly calcined at 700 to 1000 C. and pulverized for easy fabrication in the subsequent pressing step. The mixture to be pressed can be admixed with a suitable binder such as water, polyvinyl alcohol, etc.

It is advantageous that the sintered body be lapped at the opposite surfaces by abrasive powder such as silicon carbide in a particle size of 300 meshes to 1500 meshes.

The sintered bodies are provided, at the opposite surfaces thereof, with electrodes in any available and suitable method such as electroplating method, vacuum evaporation method, metallizing method by spraying or silver painting method.

'Ihe voltage variable properties are not practically affected by the kinds of electrodes used, but are alfected by the thickness of the sintered bodies. Particularly, the C- value varies in proportion to the thickness of the sintered bodies, while the n-value is almost independent of the thickness. This surely means that the voltage variable property is due to the bulk of the body, but not to the electrode.

Lead wires can be attached to the electrodes in a per se conventional manner by using conventional solder having a low melting point. It is convenient to employ a conductive adhesive comprising silver powder and resn in an Organic solvent in order to connect the lead wires to the electrodes.

Voltage variable resistors according to this invention have a high stability to temperature and to the load life test, which is carried out at 70 C. at a rating power for 500 hours. The n-value and c value do not change remarkably after heating cycles and load life test. It is advantageous for achievement of a high stability to humidity that the resultart voltage variable resistors are embedded in a humidity proof resn such as epoxy resn and phenol resn in a per se well known manner.

Presently preferred illustrative embodiments of the invention are as follows.

EXAMPLE 1 A mixture of zinc oxide and uranium oxide in a composition of Table 1 are mixed in a Wet mill for 3 hours. The mixture is dried and then calcined at 700 C. for 1 hour. The calcined mixture is pulverized by the motordriven ceramic mortar for 30 minutes and then pressed in a mold into a shape of 17.5 mm. in diameter and 2.5 mm. in thickness at a pressure of 500 kg./cm.

The pressed body is sintered in air at 1350 C. for 1 hour, and then furnace-cooled to room temperature (about 15 to about 30 C.). The sintered disc is lapped at the opposite surfaces thereof by silicon carbide in a particle size of 600 meshes. Resulting sintered disc has a size of 14 mm. in diameter and 1.5 mm. in thickness. The silver paint electrodes commercially available are attached to the opposite surfaces of sintered disc by painting. Then lead wires are attached to the silver electrodes by soldering. The electric characterstics of the resultant resistors are shown in Table 1. It will be readily understood that the Zinc oxide sintered body incorporated with uranium oxide in an amount of 0.05 to 10.0 mole percent is available for the voltage variable resistor. Particularly in the addition of uranium oxide in an amount of 0.1 to 3.0 mole percent makes the voltage nonlinear property more excellent.

TABLE UOz (mol. Uoz (mol.

percent) C (at 1 ma.) n percent) C (at 1 ma.) n

EXAMPLE 2 Starting materials composed of 99.5 mole percent of zinc oxide and 0.5 mole percent of uranium oxide are mixed, dried, calcined and pulverized in the same manner as those of Example 1. The pulverized mixture is pressed in a mold into a disc of 17.5 mm. in diameter and 5 mm. in thickness at a pressure of 500 kg./cm.

The pressed body is sintered in air at 1350 C. for 1 hour, and then furnace-cooled to room temperature. The sintered disc is ground at the opposite surface thereof into the thickness shown in Table 2 by silicon carbide in a particle size of 600 meshes. The ground disc is provided with the electrodes and lead wires at the opposite surfaces in a manner similar to that of Example 1. The electric characteristcs of the resultant resistors are shown in Table 2; the C-value varies approximately in proportion to the thickness of the sintered disc while the n-value is essentially independent of the thickness. It will be readily realized that the voltage nonlinear property of the resistors are attributed to the sintered body itself.

TABLE 2 Thickness (mm.) C (at 1 ma.) n

EXAMPLE 3 lts in a remarkably excellent voltage non- TABLE 5 3,699,058 5 o TABLE 3 as addtve resu 31203 mol lnear property.

percent) C (at l ma.)

UO (mol. percent) 11042101010641096 &3 3 4 4 3 3 &4 4 4444434 percent) EXAMPLE 6 Zinc oxide oxide containing the additions of Table 6 is fabricated into voltage variable resistors by the same TABLE 6 percent) process as that of Example 1. The electrical properties of the resulting resistors are shown in Table 6. It will be easily realized that the combination of uranium oxide and bsmuth oxide with cobalt oxide as addtives results in a remarkably excellent n-value and at the same time in a lower C-value.

EXAMPLE 7 Zinc oxide containing the additions of Table 7 is fabricated into voltage variable resistors by the same process as that of Example 1. The resulting resistors are tested under the same conditions as those of Example 4. Table 7 shows the initial C-value and difierences in C-values voltage variable resistors by the same process as that of The load life test is carried out at 70 C. ambient temheating cycle test is carried out by repeating S times the temperature for 30 minutes, cooled rapidly to -20 C.

readily realized that the combination of uranium oxide cycle in which said resistors are kept at 85 and then kept at such temperature for 30 minutes.

resistors before and after the load life test.

and calcium oxide as additive is effective environmental stability.

Load life test Heating cycle test;

AC An AC An (percent) (percent) (percent) (percent) and n-values after the load life test. It can be easily realized that the initial C-value of the resistor is lowered and at the same time the stability for the electrical and 5 environmental load life tests is excellent by using the combination of uranium oxide, bsmuth oxide and calcium oxide as an additive.

TABLE 7 CaO C (mol. t percent) 1 ma.)

perserit) (mol. percent) 5555 b bo EXAMPLE 5 Zinc oxide containing the additions of Table 5 is fabricated into voltage variable resistors by the same procedures as that of Example 1. The n-values of the result- 5 ing resistors are shown in Table 5. It will be readily seen that the combination of uranium oxide with cobalt oxide What is claimed is:

1. A voltage .variable resistor ceramic composition consisting essentially of zinc oxide and 0.05 to 10.0 mole percent of uranum oxide.

2. A Voltage variable resistor ceramic composition as claimed in claim 1 wherein said composition consists essentially of zinc oxide and 0.1 to 3.0 mole percent of uranium oxide.

3. A voltage variable resistor ceramic composition as claimed in claim 1 wherein said composition further includes 0.05 to 8.0 mole percent of one oxide selected from the group consisting of bsmuth oxide, calcium oxide and cobalt oxide.

4. A voltage variable resistor ceramic composition as claimed in claim 2 wherein said composition further includes 0.1 to 3.0 mole percent of one oxide selected from the group consisting of bsmuth oxide, calcium oxide and cobalt oxide.

5. A voltage variable resistor ceramic composition as claimed in claim 1 wheren said composition further includes 0.05 to 8.0 mole percent of bsmuth oxide and References Cited UNITED STATES PATENTS 2,258,646 10/1941 Grisdale 252-519 2,892,988 6/ 1959 Schusterus 252-518 OTHER REFERENCES Chemical Abstracts, 1964, vol. -60, 10202b.

DOUGLAS J. DRUMMOND, 'Primary Examiner U.S. Cl. X.R. 252-519, 521 

